Apparatus for the measurement of the fraction of gas in a two-component fluid flow comprising a liquid and a gas in mixture

ABSTRACT

The apparatus has two plate electrodes surrounded by a third electrode. The third electrode is maintained at an electric potential equal to half of the electric potential between the two plate electrodes. The fluid is passed between the two plate electrodes and the changes in capacitance between the two plate electrodes is measured. From this measurement the fraction of gas in the fluid is determined.

TECHNICAL FIELD

This invention relates to an apparatus for measuring the fraction of gasin a two component fluid flow comprising a liquid and a gas in mixture,in particular a flowing oil/gas mixture.

BACKGROUND ART

The fraction of gas in a two component flow of liquid and gas may bedefined as the volume of gas in an arbitrary section of a tube dividedby the volume of that section of the tube. The fraction of gas maytherefore be expressed as a number varying from zero (when the flow isentirely comprised by the liquid) to one (when the flow solely consistsof gas).

While it has been of great importance to know how much of the productionfrom an oil well is comprised by oil and how much of it is gas,measurement of the fraction of gas has always been considered with greatinterest within the offshore oil business.

Measuring devices whose operating principle is based on the detection ofchanges in capacitance, have been increasingly employmed for themeasurement of the fraction of gas in two component fluid comprisingliquid/gas mixtures.

The principle for such a measuring method is well known and amountsessentially to measuring the electrical capacitance across twoelectrodes, between which the mixture of the two components is flowing.If the area and the mutual separation of the electrodes are fixed, themeasured capacitance will be related to the fraction of gas in themixture between the electrodes.

By employing such a technique, it is in principle possible to buildinstruments with rapid dynamic response. The instrument may beconstructed in such a way as to give a non-intrusive method ofmeasurement. In spite of the advantage which is achievable in thismanner, commercially available gas fraction meters based on thecapacitance principle are relatively scarce.

The main disadvantage of known capacitance gas fraction meters is thedependence of the instrument calibration upon the nature of the flowregime being monitored. Thus, by example, the calibration curve requiredfor a bubble flow will deviate from that of an stratified flow.

One of the reasons for this flow regime dependency is the missinghomogenity of the electric field through the sample volume. If theelectric field varies through the whole sample volume, the measuredchange in capacity which arises from a change in the gas fraction, willdepend upon where the gas is located.

DISCLOSURE OF INVENTION

The object of the present invention is to remove these disadvantages andto provide a gas fraction meter of the above mentioned kind in which ahomogeneous electric field is maintained within the sample-volume of thesensor. In addition it is also an object of the present invention toprovide a non-intrusive measuring device of sturdy and simpleconstruction, by which an output voltage proportional to the fraction ofgas is generated by means of a simple signal processing circuit.

These objects are accomplished by designing an apparatus for themeasurement of the fraction of gas in a two component fluid flowcomprising a liquid and a gas in mixture, in particular a flowingoil/gas-mixture, said apparatus being based on the measurement ofalterations of the electric capacitance across two electrodes (A, B)working at mutually different potentials, between which the twocomponent mixture is forced to flow, which electrodes (A, B) form anintegral paprt in a primary sensor, where the apparatus moreoverincludes a signal processing unit, characterized by comprising inaddition to the said electrodes (A, B), a third electrode (C), which isarranged to be kept at a potential (V₁ /2), which potential is at leastapproximately equal to the potential occuring half way between the othertwo electrodes (A, B), which third electrode (C) by example is fed by asimple voltage divider and a buffer amplifier (A₂), whereby the signalprocessing unit preferably comprises a sine wave generator and a chargeamplifier (amplifier with capacitance feedback) (A₁). Besides beingcapable of fulfilling the above mentioned requirements, experiments haveshown the gas fraction measuring device not to be as dependant of theflow regime as known gas fraction measuring devices comprising only twoelectrodes.

BRIEF DESCRIPTION OF DRAWINGS

The measuring device according to the invention will be explained inmore detail in the following with reference to the drawing, where:

FIG. 1 shows a gas fraction meter comprising two main components, namelya primary sensor, shown schematically, and a signal processing unitillustrated as a circuit diagram.

FIG. 2 illustrates the distribution of the equipotential lines insidethe primary sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, the primary sensor comprises three electrodes,namely two plate electrodes A and B in parallel together with a thirdelectrode C, which, shaped like a tube, encloses the other twoelectrodes A, B. The two component flow, for example in the form of anoil/gas mixture, which is going to be monitored, passes through betweenthe parallel plate electrodes A, B and thereby causes alterations in themeasured capacitance between A and B.

This capacitance is measured by means of a sine wave generator and anamplifier A₁ with capacitance feedback; better known today by the term"charge amplifier". Because the input of this amplifier is kept clampedon "virtual earth", any alteration in the leakage capacitance to earthwill have a very little influence on the result. This makes it possibleto employ long screened cables between sensor and amplifier (if this isnecessary), without reducing the accuracy of the measurement in anysubstantial degree. If the feedback capacitance of the charge amplifierA₁ is fixed and the amplitude V₁ of the sinewave generator is keptconstant, the amplitude V₂ of the output signal from the amplifier willbe directly proportional to the alterations in the capacitance of theprimary sensor. The output voltage of the charge amplifier is in otherwords a measure of the fraction of gas in the mixture being monitored.

In order to maintain a homogeneous electric field within the samplevolume of the primary sensor, the electrode C must be kept at potentialequivalent to the potential occuring half way between the electrodes Aand B. This can be accomplished by using a simple voltage divider and abuffer amplifier A₂.

FIG. 2 illustrated the distribution of the equipotential lines insidethe primary sensor, where the electrode A is kept at a potential V₁, theelectrode B at a potential equal to 0 and the electrode C accordingly ata potential equal to V₁ /2. As shown by the equipotential lines in FIG.2, the electrical fields is homogeneous inside the sample region of theprimary sensor.

I claim:
 1. An apparatus for measuring the fraction of gas in a fluidflow comprising a gas and a liquid, said apparatus comprising:(a) afirst electrode; (b) a second electrode positioned such that said fluidflow passes between said first and second electrode; (c) a thirdelectrode surrounding said first and second electrode; (d) a sine wavegenerator and an operational amplifier with capacitance feedbackconnected to said first and second electrode for supplying an electricfield to said first and second electrode thereby establishing acapacitance between said first and second electrode; (e) a voltagedivider and a buffer amplifier that is connected to said sine wavegenerator and to said third electrode for keeping said third electrodeat an electric potential equal to the electrical potential occuringabout half way between said first and second electrode; and (f) meansfor detecting changes in the capacitance between said first and secondelectrode thereby measuring the fraction of gas in said fluid flow. 2.The apparatus of claim 1 wherein said first and second electrode areplate electrodes and said third electrode is a cylindrically shapedelectrode.
 3. An apparatus for measuring the fraction of gas in a fluidflow comprising a gas and a liquid, said apparatus comprising:(a) acylindrical electrode; (b) a first plate electrode, positioned insidesaid cylindrical electrode; (c) a second plate electrode, positionedinside said cylindrical electrode such that said fluid flow passesbetween said first and second plate electrode; (d) a sine wave generatorand an operational amplifier with capacitance feedback connected to saidfirst and second electrode plate for supplying said first and secondelectrode with an electrical signal V₁ thereby establishing acapacitance between said first and second plate electrode; (e) a voltagedivider and a buffer amplifier that is connected to said sine wavegenerator and to said cylindrical electrode for keeping said cylindricalelectrode at a potential of about V₁ /2; and (f) means for detectingchanges in said capacitance between the first and second electrode platethereby measuring the fraction of gas in the fluid flow.
 4. An apparatusfor measuring the fraction of gas in a fluid flow comprising a gas and aliquid, said apparatus comprising:(a) a cylindrical electrode; (b) afirst plate electrode positioned inside said cylindrical electrode; (c)a second plate electrode positioned inside said cylindrical electrodeand opposite said first plate electrode such that said fluid flow passesbetween said first and second plate electrode; (d) a sine wave generatorand an operational amplifier with capacitance feedback connected to saidfirst and second plate electrode for supplying an electric field betweensaid first and second plate electrode such that said first plateelectrode is at an electric potential of about V₁ and said second plateelectrode is at an electric potential of about 0; (e) a voltage dividerand a buffer amplifier that is connected to said sine wave generator andto said cylindrical electrode for keeping said cylindrical electrode ata potential of about V₁ /2; and (f) means for detecting fluctuations insaid electric field between said first and second plate electrodethereby measuring the fraction of gas in said fluid flow.